Electronic devices called “chip-on-board light-emitting diodes” (COB-LEDs) comprise an LED or LED array mounted on a printed circuit board and protected by an overlying transparent lens or other element, usually molded from a synthetic resinous material, which may serve to direct and focus the light. It is of utmost importance that the protective element (hereinafter referred to, for convenience, as the lens) of a COB-LED device remain optically transparent for an extended period of time; indeed, it is the durability of the lens that often determines the useful life of the device. The cured resin must therefore resist degradation from exposure to radiation emitted from the LED itself and other causes, such as outdoor environmental effects.
Unlike LED arrays used in calculator displays and the like, many potential applications for COB-LED devices require extraordinary brightness and entail the use of relatively high electrical currents. The corresponding levels of heat generated often make temperature management a primary design and engineering consideration, and give rise to a need for a lens-forming resin that is capable of resisting high-temperature thermal degradation and distortion.
Attempts to use one-part, radiation-cured urethane, epoxy, and rubber acrylates, and formulas based on cationic epoxies and vinyl ethers, in the manufacture of COB-LED products have, as far as is known, been unsuccessful due to unacceptable degradation of the cured resins at elevated temperatures, over time. For example, in a test procedure involving exposure for four days to a temperature of 140° C., most standard urethane acrylates are found to turn dark brown; cationic epoxies turn dark yellow, and even the better performing urethane, epoxy, and rubber acrylates take on a pronounced yellow coloration under the conditions described; and in any event light transmission is usually reduced significantly.